Refractory linings employed in basic oxygen process steel making furnaces must have sufficient strength to support the charge of molten metal in the vessel. In addition, the lining must be resistant to erosion or abrasion caused by the molten metal and molten slag. The porosity of the coked refractory composition is important with respect to the useful life of the lining in the vessel; low porosity refractory linings are relatively impervious and thus the molten metal or molten slag do not readily penetrate the surface of the lining and erode the refractory.
In order to withstand the slagging conditions found in present day basic oxygen steel furnaces, refractory linings must have a porosity after coking of less than 15% and preferably less than 13% (as determined by ASTM procedure C-607). It is also essential that the refractory shapes used for such linings remain strong from room temperature up to a temperature at which the carbonaceous bonding material begins to coke (approximately 850.degree.-950.degree. F). If a weakness occurs at a temperature below the temperature at which the coke bond begins to form, a soft zone will be created in the lining, resulting in spalling during initial heat-up of the vessel. After coking, the refractory shapes to be useful in basic oxygen furnaces, must have a transverse strength of at least 500 psi.
The conventional method of producing shaped refractory articles for use as linings in basic oxygen blowing steel making vessels involves hot forming of pitch bonded refractory bricks. In this hot forming process, both the pitch and the refractory aggregate are heated to a temperature at which the pitch becomes liquid. The aggregate and the liquid pitch are thoroughly mixed and, at an elevated temperature, fed into a mold and pressed to the desired shape. The formed shape may be either cooled in the mold or further heated to remove the lighter hydrocarbon distillates.
The hot forming process has many disadvantages. All of the process steps must be carried out at an elevated temperature in order to maintain the pitch in a fluid or liquid state. The handling and processing of the hot sticky mixture requires special equipment which is both expensive and costly to operate.
The hot forming process also results in the volatilization of coal tar distillates. These distillates are toxic to operators. Recent listings of threshold limit values of toxic materials by the American Conference of Governmental Industrial Hygienists include coal tar pitch volatiles as human carcinogens and limit exposure to 200 parts per billion.
As is apparent, there is a need for a technique of obtaining bonded refractory aggregates useful in basic oxygen process steel making vessels which avoids the numerous shortcomings of the conventional hot pitch forming process. A technique is particularly needed which does not create hazardous working conditions during processing caused by the volatilization of coal tar distillates.
Several methods have been suggested for manufacturing pitch bonded and pitch containing bricks at room temperature to avoid the high capital expenditures and other disadvantages inherent in the hot pitch bonding process. For example, British Pat. No. 690,859 discloses preparing a ramming mix or brick from a refractory aggregate and a high melting powdered pitch, which involves adding a lubricating liquid to the refractory aggregate-powdered pitch admixture. The lubricant makes the surfaces of the pitch particles oleaginous thereby permitting consolidation of the mixture at room temperatures. The addition of the disclosed liquid hydrocarbon lubricants (kerosene, benzene, or creosote) is indicated to increase the density of the product.
U.S. Pat. No. 3,285,760 to Hildinger et al also discloses a ramming composition of refractory aggregates and powdered pitch. Hildinger suggests the use of anthracene oil or heavy oil as a solvent or lubricant for the powdered pitch. Hildinger indicates, by the use of the solvent, the storage properties of the admixture are improved and that good rammed densities are obtained.
U.S. Pat. No. 3,415,667 to Cummings discloses a technique for preparing pitch bonded refractory compositions in which a liquid hydrocarbon solvent is used to liquefy the pitch. The liquefied pitch is admixed with and encapsulates coarse aggregates. Subsequently, fine aggregates are combined with the pitch encapsulated coarse aggregates and the mixture is pressed into refractory shapes. The refractory shapes are heated to remove the solvent and to solidify the pitch into a strong pitch bond. The liquid hydrocarbon solvent employed by Cummings to dissolve or liquefy the pitch is preferably a trichlorinated hydrocarbon such as trichloroethylene or trichlorobenzene. These solvents are selected because they have a boiling point between 80.degree. C. and 200.degree. C. which facilitates their removal during heating.
None of the low temperature processes produced above is proven to be entirely satisfactory. For example, none of these methods produces a brick having a coked porosity of under 15%. Also, the suggested solvents and lubricants contain toxic volatiles and therefore do not solve the problem of hazardous conditions during manufacture and use. Other suggested methods of producing, at room temperature, pitch containing brick utilize furfural, furfuryl alcohol, furan resins, or combinations thereof. For example, U.S. Pat. No. 3,496,256 to Boquist describes refractory articles made at room temperature using a pre-polymerized furfuryl alcohol in combination with powdered pitch and a catalyst. Similarly, British Pat. No. 1,268,997 discloses a binder for refractories comprising pitch and a monomeric polymerizable thermosetting admixture including furfural and phenol, cyclohexanone, or a methyl aliphatic ketone.
Although such techniques permit the manufacture at room temperature of refractory bricks having reasonably good properties, these techniques still have some rather substantial drawbacks. For example, furfural is highly toxic and furfuryl alcohol is moderately toxic. In addition, both have theshold limit values of 5 parts per million. The binder of the British patent has an extremely strong odor which is very objectionable to operators. In addition, it has been found that when using the bonding system of this patent, extreme cracking occurs, leading to a very high production loss, if the bricks are placed through a normal refractory brick drier operating at about 300.degree. F. In addition, the bricks have very low strength at temperatures of about 300.degree. F. which can result in a substantial portion of the bricks spalling off during the initial heating of a basic oxygen converter lining. The spalling can be obviated by drying very slowly at higher temperatures but such a drying process requires special equipment necessitating added operating costs and additional capital expenditures. In addition, the bonding system is very expensive.
As is apparent, there is a need for a technique for producing pitch bonded refractory shapes which can be molded at room temperatures. A desirable technique should produce good quality brick, while eliminating hazardous fumes and pungent odors irritating or injurious to operators.
It has been found, in accordance with the present invention, that refractory aggregates, bonded with a combination of a 150.degree. C. melting point coal tar powdered pitch, an aqueous solution of a water soluble binder, and an organic solvent solution of a thermosetting resin permits the formation of refractory shapes which have good density, and excellent strength as pressed, after drying, and during and after coking. The aggregate-binder mixture can be dried without bloating or cracking and refractory shapes may be produced which have a porosity after coking of less than 15%. Refractory shapes bonded with such a binder system maintain their high strength through all temperature ranges up to the operating temperature of basic oxygen process steel furnaces. The bonding system may be used with conventional refractory processing equipment, thereby eliminating high capital expenditures. Refractory shapes can be molded at room temperature, without the evolution of hazardous and objectionable odors. The combination of aqueous solution of a water soluble binder and an organic solvent solution of a thermoplastic polymer results in refractory articles having strengths after coking considerably in excess of what would be predicted. The strength of refractory articles bonded with such a system is generally more than double than that obtained when either material is used alone.